The present invention relates to an electroluminescence (hereinafter abbreviated to EL) display panel having a layer-built structure containing phosphor and dielectric layers, and in particular to an EL display panel having a structure which is optimized to provide high display brightness with low power consumption, and is suited for use as a flat panel display having a high degree of resolution, for office automation equipment, computer terminals, etc.
An EL display panel emits light in response to an applied AC electric field, and is made up of a phosphor layer having a dielectric layer formed on one or on both sides thereof, with the layered structure thereby formed being sandwiched between an array of elongated mutually intersecting data electrodes and scanning electrodes, to thereby define an array of display elements. With one method of driving such a display panel (referred to in the following as the field-refresh drive method), periodically repetitive scanning drive of these electrodes is executed such that a voltage V.sub.ON (=V.sub.H + .DELTA.V or higher) is applied once in each scanning (field) interval to each display element which is to be selected (i.e. is to be set in the light-emitting state), and a voltage V.sub.OFF (=V.sub.H -.DELTA.V or less) is applied to each non-selected display element (i.e. which is to be left in the non-emitting state). Upon completion of scanning of the entire display, a refresh pulse V.sub.R having a polarity that is opposite to that of the voltage (V.sub.H +.DELTA.V) is applied to all of the display elements, to thereby provide AC drive operation. Voltage V.sub.H is a threshold voltage level, at which emission of light begins, while .DELTA.V is a modulation voltage which serves to determine the elements which are selected and non-selected, i.e. the elements which emit light and the elements which do not. With this drive method, each time a scanning electrode is selected during the sequential scanning, the address data for the data electrodes are updated and a data pulse is generated.
The electrical power which is required to drive such an EL display panel consists of a modulation drive component, a component corresponding to the threshold voltage V.sub.H required to initiate the emission of light, and a component corresponding to the refresh voltage V.sub.R. The actual values of the drive voltages .DELTA.V, V.sub.H and V.sub.R are determined by the light emission characteristics of the EL display panel.
FIG. 1 illustrates the relationship between emitted light brightness and applied voltage, for an EL display panel, and shows V.sub.H, .DELTA.V, V.sub.R and examples of voltages V.sub.ON and V.sub.OFF respectively utilized for selection and non-selection of display elements. Generally speaking, the values of V.sub.ON and V.sub.OFF are determined by the brightness or luminance of the display and the uniformity of that display brightness. These depend upon the thickness and the quality of the data electrode and the phosphor layer of the EL display panel. Ideally, the brightness of emission from a display element should rise sharply in response to variation of the voltage applied to that element (i.e. within the range V.sub.OFF to V.sub.ON shown in FIG. 1), in order to enable the value of .DELTA.V to be made as small as possible. In the prior art, efforts to achieve this ideal form of operation have been directed mainly towards research into enhancement of the light-emission efficiency of the EL display panel. As an alternative approach to this problem, several drive methods have been proposed for such an EL display panel. However in order to optimize the operation of an EL display panel, i.e. to attain a high level of display brightness with minimum power consumption, it is necessary to consider both the configuration of the elements of the EL display panel, and the drive method. None of the EL display panels which are being marketed at the present time have been produced on the basis of such a design philosophy. As a result, such prior art EL display panels present severe problems with regard to excessive power consumption, if it is attempted to produce a large-scale high-definition display panel.
With regard to the power consumption in the case of the field-refresh drive method described above, since the display elements each have electrical capacitance, the power consumption can be computed as the amount of power which is required to execute charging and discharging of the capacitances of these elements. This power consumption will vary in accordance with the display pattern which is produced by the display. The display pattern which results in maximum power consumption will vary, depending upon the particular drive method which is utilized. In general, each of the data electrodes of the display panel is driven by a corresponding drive transistor, and in the case of the field-refresh drive method the maximum level of power consumption occurs when all of the data drive transistors act to discharge all of the display elements, after all of the display elements have been charged to the modulation voltage .DELTA.V. Designating this maximum value of power consumption under such a drive condition as P.sub.M, then the value of P.sub.M for a thin-film EL display panel is given by the following equation, from the electrical capacitance A.multidot.C.sub.T of the entire display area (where A is the display area and C.sub.T is the electrical capacitance of the display panel per unit of area), the voltages .DELTA.V, V.sub.H and V.sub.R which are applied during the drive process, the number of data electrodes M, the number of scanning electrodes N, the total stray capacitance C.sub.o of the drive lines (including the output capacitance of the drive transistors), and the field frequency F: EQU P.sub.M =A.multidot.F(2N.multidot.C.sub.T .multidot..DELTA.V.sup.2 +C.sub.t .multidot.V.sub.H.sup.2 +C.sub.T .multidot.V.sub.R.sup.2)+N(M+N-1).multidot.C.sub.o .multidot.F.multidot.V.sub.H.sup.2 ( 1)
The derivation of equation (1) is given by Yoshiharu Kanaya, Hiroshi Kishishita, and Jun Kawaguchi in "Nikkei Electronics" of 2nd Apr. 1979, in pages 118 to 142.
If the values of the drive voltages .DELTA.V, V.sub.H and V.sub.R are established in accordance with the light emission characteristic of the EL display panel and the electrical capacitance A.multidot.C.sub.T of the entire display area and the display element configuration, then the power consumption can be immediately derived from equation (1) above, based on the size of the EL display panel, the numbers of scanning electrodes and data electrodes M and N, and the field frequency F (the latter being sometimes referred to as the frame frequency).
In the prior art, the display element configuration of an EL display panel has been determined by a process of trial and error, based upon a desired value of display brightness, the number of display elements of the display, the size of each display element, the power consumption, and limitations of drive voltage. As a result, it has not been possible in the prior art to minimize the power consumption of an EL display panel. Furthermore, as the size of the display area of such an EL display panel is increased, problems arise with regard to the necessity for reducing power consumption and for shortening the charging time of the display elements.